The Emission-Line Spectra of Major Mergers: Evidence for Shocked Outflows

TL;DR

This study analyzes the emission-line spectra of ULIRG mergers to identify shock-driven outflows caused by starburst activity, revealing their physical properties and potential for application in high-redshift galaxy observations.

Contribution

It demonstrates that shock-like emission in ULIRGs originates from starburst-driven outflows and introduces a spectral decomposition method to analyze such features.

Findings

Shock-like emission indicates starburst-driven outflows.

Broad, shocked emission is spatially extensive and blueshifted.

Shocks account for a significant fraction of supernova energy in winds.

Abstract

Using a spectral decomposition technique (Soto & Martin 2012, hereafter Paper I), we investigate the physical origin of the high-velocity emission line gas in a sample of 39 gas-rich, ultraluminous infrared galaxy (ULIRG) mergers. Regions with shock-like excitation were identified in two kinematically distinct regimes, characterized by broad ($\sigma >$ 150 \kms) and narrow linewidths. Here we investigate the physical origin of the high-velocity (broad) emission with shock-like line ratios. Considering the large amount of extinction in these galaxies, the blueshift of the broad emission suggests an origin on the near side of the galaxy and therefore an interpretation as a galactic outflow. The large spatial extent of the broad, shocked emission component is generally inconsistent with an origin in the narrow-line region of a AGN, so we conclude that energy and momentum supplied by the starburst drives these outflows. The new data are used to examine the fraction of the supernova energy radiated by shocks and the mass loss rate in the warm-ionized phase of the wind. We show that the shocks produced by galactic outflows can be recognized in moderately high-resolution, integrated spectra of these nearby, ultraluminous starbursts. The spectral fitting technique introduced in Paper I may therefore be used to improve the accuracy of the physical properties measured for high-redshift galaxies from their (observed frame) infrared spectra.